The present invention relates generally to electrical power supplies. More particularly, it relates to switching power converters and a circuit for driving secondary-side switches with energy from the primary side.
Synchronous rectifier DC/DC converters typically have a primary circuit and a secondary circuit coupled through a transformer. The secondary circuit has switches that rectify the power coupled through the transformer. Such power converters are commonly used to provide the low-voltage, high-current power required for operating microprocessors and the like from higher voltage power sources.
In recent years, successive generations of microprocessors have required power with decreased voltage, increased current, decreased ripple and increased current slew rate. For example, some microprocessors currently in development will require 100 Amps at 1 volt with less than 25 mV ripple. Additionally, circuitboard space is limited, and the high cost of large capacity filtering components (e.g. capacitors, inductors) must be avoided. In order to meet these requirements, power converters must operate at higher frequencies. However, higher frequency operation results in greatly reduced power efficiency. Specifically, increasing frequency results in higher switching losses (e.g. reverse recovery loss), higher gate driving losses and higher body diode conduction losses. To make high frequency operation possible, and hence provide power converters for future microprocessors, these losses must be reduced.
Several methods for reducing gate drive losses and for self-driving secondary gates are known in the art. For example, secondary switches can be self-driven by a cross coupled secondary circuit design. However, the cross-coupled secondary design does not function properly for very low output voltages because there is insufficient voltage to drive the switch gates.
Another known self-driving technique is described by Pedro Alou et al. in xe2x80x9cA new self driving scheme for synchronous rectifiers: single winding self-driven synchronous rectificationxe2x80x9d, published in IEEE Transactions On Power Electronics, Vol. 16, No. 6, November 2001. In this technique, secondary side switches are driven by a circuit powered from an additional winding on the main transformer. A problem with this technique is ringing in the gate drive signal, which tends to turn off the secondary switches at inappropriate times. Also, it requires an additional winding on the transformer, which complicates the transformer design. Further, it tends to result in increased body diode conduction loss.
It would be an advance in the art of electrical power conversion to be able to reduce gate drive losses such that higher frequency operation of power converters is practical. It would be particularly advantageous to reduce gate drive losses using a self-drive scheme that does not produce ringing and unwanted noise in the secondary circuit, and provides reduced body diode conduction loss.
The present invention includes a power converter with a primary circuit, a secondary circuit, and a driver for driving switches in the secondary circuit. The driver is connected to, and obtains power from, a swing node in the primary circuit. The driver circuit includes an isolation device (e.g. drive transformer, piezotransformer, optoisolator) so that the primary circuit and secondary circuit are electrically isolated.
The primary circuit can be a full bridge circuit, an active clamp forward circuit, or an asymmetrical half bridge circuit, for example. The secondary circuit can be a half-wave circuit, a center-tapped circuit, or a current-doubler circuit, for example.
The driver circuit can have a damping resistor for preventing voltage oscillations at a gate of the secondary side switch. The damping resistor preferably has a small resistance, for example, less than 5 Ohms, or 1 Ohm. If a drive transformer is used, it preferably has a small leakage inductance, for example, less than 10 or 5 nanohenries.
The driver circuit can control two secondary side switches.